Chip type varistor and method of manufacturing the same

ABSTRACT

There is provided a varistor having high surge resisting capability in spite of its small size and a method of manufacturing the same. The varistor is mainly composed of zinc oxide and contains a composite oxide expressed by the chemical formula Zn 2 SnO 4 . It is manufactured using a method wherein zinc oxide and tin oxide are mixed; the mixture is subjected to a thermal process thereafter to obtain a composite oxide expressed by the chemical formula Zn 2 SnO 4 ; and the composite oxide is combined with the zinc oxide which is the main component and a thermal process is performed to obtain a raw material for the varistor.

BACKGROUND OF THE INVENTION

[0001] This is a divisional of U.S. patent application Ser. No.09/023,683, filed Feb. 13, 1998 in the names of Kazuhiro KANEKO andKazutaka NAKAMURA.

[0002] 1. Field of the Invention

[0003] The present invention relates to a varistor (variable resistor),specifically to a varistor mainly composed of zinc oxide and a method ofmanufacturing the same.

[0004] 2. Description of the Related Art

[0005] A varistor is a circuit element whose resistance abruptlydecreases when the voltage applied to the element exceeds apredetermined level. On the other hand, the resistance is extremelylarge when the applied voltage is lower than that level. Because of suchcharacteristics, varistors are used for protecting semiconductorelements from a surge voltage, for example.

[0006] Resistance elements referred to as zinc oxide varistors whichutilize voltage/current non-linearity are mainly composed of zinc oxideand are manufactured using a method wherein zinc oxide is combined witha plurality of additives, pulverized and mixed; the resultant fineparticles are calcined to produce a ceramic raw material powder forvaristors; and a body molded from the ceramic raw material powder isfired to provide varistor elements. Energy barriers are formed inboundary barrier layers in such a varistor element due to the presenceof impurity energy levels formed at boundaries between zinc oxideparticles, which results in excellent voltage/current non-linearity.

[0007] The voltage of a varistor element at the time when it exhibitsthe voltage/current non-linearity is referred to as the “varistorvoltage”, and the voltage that appears on an element when a current of 1mA flows through the element is generally used as such a varistorvoltage. Such a voltage is normally represented by “V_(1 mA)”.

[0008] The varistor's function of absorbing surge noises andelectrostatic noises are attributable to its excellent voltage/currentnon-linearity. The surge noise absorbing capability of a varistor isevaluated using a current value at which the varistor element is brokenwhen a sequentially increased surge current is applied to the varistorelement. It serves as an important index that indicates the durabilityof a varistor element.

[0009] A varistor mainly composed of zinc oxide has excellent surgeresisting capability which increases in proportion to the area of itselectrode. However, no compact varistor element capable of resistingsurge noises of 3000 A/cm² or more has been provided.

[0010] Meanwhile, there is an accelerated trend toward smallerelectronic apparatus, e.g., the spread of mobile communicationapparatus, which is accompanied by significant trend toward smallerelectronic components and devices of various types. While this hasresulted in an increased demand for smaller varistors, a reduction inthe size of the varistor leads to a reduction in the effective area ofthe electrode thereof. For this reason, a need has arisen for varistorelements having improved surge resisting capability per unit area.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of the present invention to provide avaristor having high surge resisting capability in spite of a reducedsize and a method of manufacturing the same.

[0012] According to a first aspect of the invention, there is provided aceramic composition characterized in that it is mainly composed of zincoxide and contains a composite oxide expressed by the chemical formulaZn₂SnO₄.

[0013] According to a second aspect of the invention, there is provideda ceramic composition characterized in that it contains the compositeoxide expressed by a chemical formula Zn₂SnO₄ in an amount in the rangefrom about 0.01 to 20 mol %.

[0014] According to a third aspect of the invention, there is provided aceramic composition characterized in that it is mainly composed of zincoxide and contains bismuth oxide, cobalt oxide, manganese oxide and acomposite oxide expressed by the chemical formula Zn₂SnO₄.

[0015] According to a fourth aspect of the invention, there is provideda ceramic composition characterized in that it contains about 0.10 to 2mol % bismuth oxide, about 0.10 to 2 mol % cobalt oxide, about 0.10 to 2mol % manganese oxide and about 0.01 to 20 mol % Zn₂SnO₄.

[0016] According to a fifth aspect of the invention, there is provided avaristor characterized in that it comprises a layered body formed by aplurality of ceramic layers mainly composed of zinc oxide containing acomposite oxide expressed by the chemical formula Zn₂SnO₄, an innerelectrode layer interposed between the ceramic layers of the layeredbody and an outer electrode formed on the surface of the layered bodyand electrically connected to the inner electrode layer.

[0017] According to a sixth aspect of the invention, there is provided avaristor characterized in that it contains the composite oxide expressedby the chemical formula Zn₂SnO₄ in an amount from about 0.01 to 20 mol%.

[0018] According to a seventh aspect of the invention, there is provideda varistor characterized in that it comprises a layered body formed by aplurality of ceramic layers mainly composed of zinc oxide containingbismuth oxide, cobalt oxide, manganese oxide and a composite oxideexpressed by the chemical formula Zn₂SnO₄, an inner electrode layerinterposed between the ceramic layers of the layered body and an outerelectrode formed on the surface of the layered body and electricallyconnected to the inner electrode layer.

[0019] According to an eighth aspect of the invention, there is provideda varistor characterized in that it contains about 0.10 to 2 mol %bismuth oxide, about 0.10 to 2 mol % cobalt oxide, about 0.10 to 2 mol %manganese oxide and about 0.01 to 20 mol % Zn₂SnO₄.

[0020] According to a ninth aspect of the invention, there is provided amethod of manufacturing a varistor characterized in that it comprisesthe steps of mixing zinc oxide and tin oxide, thermally processing themixture thereafter to obtain a composite oxide expressed by the chemicalformula Zn₂SnO₄, and combining the composite oxide with the zinc oxidewhich is the main component and thermally processing the combination toobtain a raw material for a varistor.

[0021] According to a tenth aspect of the invention, there is provided amethod of manufacturing a varistor characterized in that the mixingratio between the zinc oxide and tin oxide is about 2:1 in terms of molratio.

[0022] According to an eleventh aspect of the invention, there isprovided a method of manufacturing a varistor characterized in that themixture is subjected to a thermal process at 1100° C.

[0023] According to a twelfth aspect of the invention, there is provideda method of manufacturing a varistor characterized in that the compositeoxide expressed by Zn₂SnO₄ is in an amount in the range from about 0.01to 20 mol % relative to the zinc oxide which is the main component.

[0024] As described above, there is provided according to the presentinvention varistors and chip type varistors having high surge resistingcapability by adding a composite oxide expressed by Zn₂SnO₄ to the zincoxide which is the main component.

[0025] Further, by adding bismuth oxide, cobalt oxide and manganeseoxide in addition to Zn₂SnO₄, there is provided varistors and chip typevaristors which have not only improved surge resisting capability butalso has excellent electrical characteristics such as non-linearitycoefficient, electrostatic capacity and dielectric loss.

[0026] Further, according to the method of manufacture wherein zincoxide and tin oxide are mixed; the mixture is subjected to a thermalprocess thereafter to obtain a composite oxide expressed by the chemicalformula Zn₂SnO₄; and the composite oxide is added to the zinc oxidewhich is the main component and a thermal process is performed to obtaina raw material for a varistor, there is provided a raw material for avaristor which exhibits high surge resisting capability to allowmanufacture of a reliable varistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1A is a graph showing the relationship between the amount ofbismuth oxide added and the non-linearity coefficient according to thepresent invention.

[0028]FIG. 1B is a graph showing the relationship between the amount ofcobalt oxide added and the non-linearity coefficient according to thepresent invention.

[0029]FIG. 1C is a graph showing the relationship between the amount ofmanganese oxide added and the non-linearity coefficient according to thepresent invention.

[0030]FIG. 2A is a graph showing the relationship between the amount ofbismuth oxide added and the dielectric loss according to the presentinvention.

[0031]FIG. 2B is a graph showing the relationship between the amount ofcobalt oxide added and the dielectric loss according to the presentinvention.

[0032]FIG. 2C is a graph showing the relationship between the amount ofmanganese oxide added and the dielectric loss according to the presentinvention.

[0033]FIG. 3 is a sectional view of the varistor according to thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] A mode for carrying out the present invention will now bedescribed with reference to a preferred embodiment thereof.

[0035] First, zinc oxide (ZnO) having purity of 99% or more and tinoxide (SnO₂) having purity of 99% or more were mixed at a mol ratio of2:1. Any convenient ratio could have been used but it is most convenientto use a ratio close to stoichiometric. The mixture was combined withpulverizing media, pulverized and agitated in pure water, dehydrated anddried. Thereafter, the mixture was granulated and subjected to heatingat 1200° C. to produce a Zn₂SnO₄ powder. In general, temperatures of atleast about 1000 to 1300° C. can be used.

[0036] Next, ceramic raw material powders for varistors was produced.

[0037] Specifically, ceramic raw material powders for varistors wereprepared by adding bismuth oxide (Bi₂O₃), cobalt oxide (Co₃O₄) andmanganese oxide (Mn₃O₄) in predetermined amounts shown in Table 1 tozinc oxide (ZnO) having purity of 99% or more such that the componentstotaled to 100 mol % and by further adding the Zn₂SnO₄ powder. Thesamples marked with an asterisk in Table 1 are outside the scope of theinvention. TABLE 1 Sample Composition/mol % No. ZnO Bi₂O₃ Co₃O₄ Mn₃O₄Zn₂SnO₄ *1  97.50 0.50 1.00 1.00 0 2 97.49 0.50 1.00 1.00 0.01 3 97.400.50 1.00 1.00 0.10 4 97.00 0.50 1.00 1.00 0.50 5 96.50 0.50 1.00 1.001.00 6 92.50 0.50 1.00 1.00 5.00 7 87.50 0.50 1.00 1.00 10.00 8 77.500.50 1.00 1.00 20.00 *9  67.50 0.50 1.00 1.00 30.00

[0038] The prepared ceramic raw material powders for varistors werecombined with pulverizing media, pulverized and agitated in pure water,dehydrated and dried. Then, the raw material powders were granulated andcalcined and were pulverized and hydrated in pure water and dried againto provide varistor materials.

[0039] Subsequently, chip type varistors were produced from thosevaristor materials. Specifically, the varistor materials were combinedwith a binder, plasticizer and a plurality of stabilizers inpredetermined amounts in an organic solvent and were mixed to produceslurries which in turn were subjected to a doctor blade process to formgreen sheets having a thickness of about 10 μm for inner electrodelayers and green sheets having a thickness of about 40 μm for outerlayers.

[0040] Next, a conductive metal paste containing Pt was screen-printedon the surface of the green sheets to form inner electrode layers. Apredetermined number of green sheets having an inner electrode layerprinted thereon were stacked, and a predetermined number of green sheetson which the Pt conductive metal layer was absent were stacked on theupper and lower sides thereof. Thereafter, the product was bonded bypressing it in a press machine to produce a press-bonded green sheetbody in the form of a block having a thickness of about 1 mm. Then, thepress-bonded green sheet body was cut into a chip type varistor of apredetermined size.

[0041] Next, such chip type varistor elements were placed in a bakingfurnace and were baked for three hours at about 900 to 1200° C. afterremoving the binder by heating.

[0042] Thereafter, a terminal electrode made of Ag was formed at ends ofthe resultant baked chip type varistors to complete chip type varistorsand establish electrical connection to the inner electrodes. Theelectrical characteristics of the varistors were then evaluated.

[0043]FIG. 3 shows a sectional view of the varistor 1. Inner electrodes3 are formed in a ceramic layer 2 and outer electrodes 4 are appliedonto the surface of the ceramic layer.

[0044] Specifically, measurement of the voltage-current characteristicswas carried out to measure the varistor voltage (V_(1 mA)) andnon-linearity coefficient (α) and to measure electrostatic capacity(C_(p)) and dielectric loss (D.F.) at a frequency of 1 MHz and a voltageof 1.0 V.

[0045] Next, surge resisting capability was measured. Specifically, asquare wave current of {fraction (8/20)} μsec. was successively appliedfour times at intervals of five minutes, and the surge resistingcapability was the current value at the point in time when the change inthe varistor voltage after the application of the current reached 10% ofthe initial value.

[0046] Table 2 shows the results of measurement of electricalcharacteristics, i.e., the varistor voltage (V_(1 mA)), non-linearitycoefficient (α) and electrostatic capacity (C_(p)), and surge resistingcapability (Ip). The sample numbers in Tables 1 and 2 correspond to eachother. TABLE 2 Sample No. V_(1mA) α Cp (pF) D.F. (%) Ip (A/cm²) *1  6.342.1 380.3 4.20 2050 2 6.8 55.3 330.2 1.25 3050 3 6.8 54.2 328.2 1.213100 4 6.7 55.2 325.2 1.18 3150 5 6.7 55.4 324.6 1.26 3200 6 6.9 58.6323.5 1.08 3600 7 6.8 55.3 323.0 1.15 3150 8 7.1 53.2 322.8 1.87 3150*9  7.2 28.2 300.1 12.56 2500

[0047] As apparent from the above results, when the Zn₂SnO₄ powder isused as an additive to a varistor raw material, a varistor having highsurge resisting capability can be provided. On the contrary, whenZn₂SnO₄ is not added, the surge resisting capability does not reach 3000A/cm² as indicated by sample No. 1. Further, when the amount of Zn₂SnO₄added exceeds about 20 mol %, the surge resisting capability is below3000 A/cm² as indicated by sample No. 9. Further, when Zn₂SnO₄ is notadded, although the surge resisting capability is improved, it is notpreferable because the non-linearity coefficient becomes small and thedielectric loss becomes high. The non-linearity coefficient and thedielectric loss are values that reflect the states of boundary levelsand energy barriers, and deterioration of these values leads todeterioration of the performance of the varistor. Therefore, thepreferable amount of Zn₂SnO₄ added is in the range from about 0.01 to 20mol % and more preferably about 1-10 mol %.

[0048] A second embodiment of the present invention will now bedescribed.

[0049] An evaluation was made on changes in electrical characteristicsdepending on the amounts of bismuth oxide (Bi₂O₃), cobalt oxide (Co₃O₄)and manganese oxide (Mn₃O₄) under the condition wherein the amount ofZn₂SnO₄ added was kept constant, i.e., the amount of Zn₂SnO₄ added waskept at 5.00 mol %, to achieve surge resisting capability of 3000 A/cm²or more.

[0050] Specifically, varistor materials were prepared in the same manneras for the first embodiment; green sheets were formed; and chipprocessing was performed including formation of inner electrode layers,stacking, press-bonding and cutting. Outer electrodes were provided onbaked chips to provide chip type varistors whose electricalcharacteristics were evaluated.

[0051]FIGS. 1A through 1C show changes in the non-linearity coefficientdepending on the added amount of each of the additives, i.e., bismuthoxide (Bi₂O₃), cobalt oxide (Co₃O₄) and manganese oxide (Mn₃O₄), andFIGS. 2A through 2C show changes in the dielectric loss depending on theadded amount of each of the additives, i.e., bismuth oxide, cobalt oxideand manganese oxide.

[0052] As apparent from FIGS. 1A through 1C and FIGS. 2A through 2C, areduction in the non-linearity coefficient and an increase in thedielectric loss were observed when the amount of each of bismuth oxide,cobalt oxide and manganese oxide added was outside of the amountsaccording to the present invention. Therefore, the amount of each ofbismuth oxide, cobalt oxide and manganese oxide added is preferably inthe range from about 0.10 to 2 mol %.

[0053] Although the Zn₂SnO₄ powder was added before the calcination ofthe ceramic raw material powders for varistors in the above-describedembodiments, it is not essential to add it before calcination, and itwill provide the same effect of improving surge resisting capabilityeven when it is added after calcination. Further, although a conductivemetal paste of Pt was used to form inner electrode layers, mixed Ag—Pdmay be used instead. Furthermore, although the above-describedembodiments have referred to chip type layered varistors, the presentinvention is not limited to this type of varistors and can provide thesame effect when applied to single plate varistors and other types ofvaristors.

[0054] The present invention makes it possible to improve surgeresisting capability per unit area of a zinc oxide type varistor and,consequently, to provide a varistor capable of absorbing surge noises of3000 A/cm² even with a small size. Further, the present invention iseffective in improving the voltage/current non-linearity and reliabilityof a varistor. Thus, the present invention can contribute to reductionof the size of an electronic component having a varistor functionconstituted by such a varistor and to improvement of the reliabilitythereof.

[0055] While the invention has been particularly shown and describedwith reference to preferred embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of manufacturing a varistor raw materialcomprising the steps of: heating a mixture of zinc oxide and tin oxideto obtain a composite oxide expressed by the chemical formula Zn₂SnO₄;and combining said composite oxide and zinc oxide and heating thecombination.
 2. The method of manufacturing a varistor raw materialaccording to claim 1, wherein the mixture mol ratio of said zinc oxideto tin oxide is about 2:1.
 3. The method of manufacturing a varistor rawmaterial according claim 2, wherein mixture is heated at about1000-1300° C. to obtain the composite oxide and said combination isheated at about 900-1200° C.
 4. The method of manufacturing a varistorraw material according to claim 3, wherein the amount of said compositeoxide expressed by Zn₂SnO₄ is about 0.01 to 20 mol % relative to 100 mol% of said zinc oxide.
 5. The method of manufacturing a ceramiccomposition according to claim 4, wherein bismuth oxide, cobalt oxideand manganese oxide are combined with said zinc oxide and compositeoxide prior to heating.
 6. The method of manufacturing a varistor rawmaterial according claim 5, wherein said mixture is heated at about1200° C. and combination is heated at about 1000° C.
 7. The method ofmanufacturing a varistor raw material according to claim 6, wherein theamount of bismuth oxide is 0.1 to 2 mol %, the amount of cobalt oxide is0.1 to 2 mol % and the amount of manganese oxide is 0.1 to 2 mol %. 8.The method of manufacturing a varistor raw material according to claim7, wherein the amount of said composite oxide expressed by Zn₂SnO₄ isabout 1 to 10 mol % relative to 100 mol % of said zinc oxide.
 9. Themethod of manufacturing a varistor raw material according claim 1,wherein mixture is heated at about 1000-1300° C. to obtain the compositeoxide and said combination is heated at about 900-1200° C.
 10. Themethod of manufacturing a varistor raw material according to claim 9,wherein the amount of said composite oxide expressed by Zn₂SnO₄ is about0.01 to 20 mol % relative to 100 mol % of said zinc oxide.
 11. Themethod of manufacturing a varistor raw material according to claim 10,wherein bismuth oxide, cobalt oxide and manganese oxide are combinedwith said zinc oxide and composite oxide prior to heating.
 12. Themethod of manufacturing a varistor raw material according claim 11,wherein said mixture is heated at about 1200° C. and combination isheated at about 1000° C.
 13. The method of manufacturing a varistor rawmaterial according to claim 12, wherein the amount of bismuth oxide is0.1 to 2 mol %, the amount of cobalt oxide is 0.1 to 2 mol % and theamount of manganese oxide is 0.1 to 2 mol %.
 14. A method ofmanufacturing a varistor raw material comprising the steps of: heating amixture of zinc oxide and tin oxide to obtain a composite oxideexpressed by the chemical formula Zn₂SnO₄; and combining said compositeoxide and zinc oxide and heating the combination, wherein the amount ofsaid composite oxide expressed by Zn₂SnO₄ is about 0.01 to 20 mol %relative to 100 mol % of said zinc oxide.
 15. The method ofmanufacturing a varistor raw material according to claim 14, wherein theamount of said composite oxide expressed by Zn₂SnO₄ is about 1 to 10 mol% relative to 100 mol % of said zinc oxide.
 16. The method ofmanufacturing a varistor raw material according claim 15, wherein saidmixture is heated at about 1200° C. and combination is heated at about1000° C.
 17. The method of manufacturing a ceramic composition accordingto claim 16, wherein bismuth oxide, cobalt oxide and manganese oxide arecombined with said zinc oxide and composite oxide prior to heating. 18.The method of manufacturing a ceramic composition according to claim 17,wherein the amount of bismuth oxide is 0.1 to 2 mol %, the amount ofcobalt oxide is 0.1 to 2 mol % and the amount of manganese oxide is 0.1to 2 mol %.